Does It Matter Which Side of the Foil Goes on the Outside?

My grandma says to wrap potatoes in foil with the shiny side in when I bake them. What difference does it make?

Aluminum foil is run through finishing rollers in the factory in a double layer; the sides that touch the rollers end up shiny, and the other sides end up dull. The theory is that the shiny side will reflect heat waves, so if you wrap something with the shiny side facing in, when the food starts to heat up, that heat will be reflected back into the food. But in reality, it doesn’t matter. “The difference in cooking is so minimal that we really say there’s no difference,” says Pat Schweitzer, spokeswoman for Reynolds Kitchens. “In a scientific study, there might be a small difference, but there’s no need to worry about it.” The one time it actually does matter is if you’re using nonstick foil. The nonstick coating is on the dull side, so that should be the side with which food comes in contact.